1. Field of the Invention
The present invention relates to an apparatus for DNA sequencing or the like, and more particularly, it relates to a gel electrophoresis apparatus for gelelectrophoresing fluorescence-labelled samples and scanning optical systems for exciting and receiving fluorescence in a direction perpendicular to the electrophoresis direction, thereby detecting the electrophoresis pattern.
2. Description of the Background Art
Fluorescence-labelled samples are DNA fragments which are fluorescence-labelled in a primer part or a dideoxy part and prepared by the Sanger's sequencing method. A developed pattern obtained by gel-electrophoresing the fluorescence-labelled samples directly provides the DNA sequence.
FIG. 11 shows a fluorescence detection type gel electrophoresis apparatus, which is described in U.S. Pat. No. 4,811,218. Fluorescence-labelled samples are electrophoresed in an electrophoresis gel 104, which is held between glass panels to extend perpendicularly to the figure plane, along the said perpendicular direction. A stage 239 is guided by a guide rail 233, and scanned in a direction perpendicular to the electrophoresis direction, i.e., vertically in this figure, by rotation of a screw 252 which is driven by a motor 237. The stage 239 is provided with a condenser lens 260, so that a laser beam 250, which is excitation light, is reflected by a mirror 251 to enter the lens 260, and further reflected by another mirror 255 provided on the stage 239, to irradiate a portion of the electrophoresis gel 140 to be measured. Fluorescence outgoing from the measured portion is collected by a condenser lens 221 provided on the stage 239, selected by an interference filter 223 to pass through a lens 225, and detected by a photomultiplier tube 229.
In the apparatus shown in FIG. 11, the electrophoresis gel 104, the scanning direction, which is determined by the screw 252 and the guide rail 233, and the direction of incidence of the excitation light upon the condenser lens 260 must be absolutely parallel to each other. This condition must be regularly satisfied also when the gel 104 is exchanged.
FIG. 12 shows such a case that the above condition is not satisfied. In a certain scanned position A, the gel portion irradiated with the excitation light 250 is correctly located on the optical axis of the light receiving optical system, to provide a strong signal. However, if the glass panels holding the gel 104 are varied in thickness, or the gel 104 is not sufficiently parallel with the screw 252, for example, the gel portion irradiated with the excitation light 250 is displaced from the optical axis of the light receiving optical system in another scanned position B as shown in FIG. 12, whereby the signal strength is so reduced that no signal is detected in an extreme case. Thus, the signal strength is varied with the position.
The lens 225 is adapted to reduce such displacement. In this optical system, however, an angle .theta. of incidence is set at a small value of about 20.degree. to 35.degree. in order to reduce scattered light. Therefore, it is impossible to sufficiently compensate for the displacement between the excited position of the gel 104 and the optical axis of the light receiving optical system caused by variation of a distance D between the gel 104 and the stage 239 due to ununiformity of thickness of the glass panels or an error in parallelism.
In order to eliminate such displacement, it is conceivable to detect a spot position of the excitation light and move the optical axis by a negative-feedback servo mechanism. However, such a method inevitably requires a detector for the spot position, and hence the structure is complicated.